Publications and Reports “Study of the Distribution and Characterization of Forms of Fe in Particle-Size Fractions of an Etnic Haplustoll by Selective Dissolution Techniques, X-Ray Diffraction and Mössbauer Spectroscopy” S.G. Acebal, M.E. Aguirre, R.M. Santamaría, A. Mijovilovich, S. Petrick, C. Saragovi Hyperfine Interactions 148/149 (2003) 3-12 Aggregate stability depends on soil properties such as organic matter content, type and quantity of clay and poorly crystalline Fe and Al oxide and oxy-hydroxides. Then, the quantification and characterization of forms of Fe and Al are of major importance for understanding soil structure. Particle-size fractions (5-2 m, 2-1 m, and 1 m) from the Ap horizon of an Entic Haplustoll from Argentina, treated with the selective-dissolution techniques ammonium oxalate (OX), dithionite-citrate-bicarbonate (DCB), NaOH, and Napyrophosphate (PY), are studied by X-ray powder diffraction (XRD) and 57Fe Mössbauer spectroscopy (MS). Quartz, feldspar, smectite, illite and interstratified illite- smectite are predominant whereas Fe and Al oxides and oxy-hydroxides are present in low concentration. Al- substituted hematite and poorly crystallized hematite and goethite are present in similar proportions. “Temperature Evolution of Crystal Field Interactions Across the Jahn-Teller Transition in a La7/8Sr1/8MnO3 Single Crystal” G. Alejandro, M.C.G. Passeggi, D. Vega, C.A. Ramos, M. T. Causa, M. Tovar, R. Senis Physical Review B68 (2003) 214429 We present ESR, x-ray diffraction and dilatometry studies in a La7/8Sr1/8MnO3 single crystal. ESR experiments were performed in the paramagnetic regime (TC ~ 180 K), from 220 K to 570 K at a microwave frequency of 9.4 GHz. We measured the ESR linewidth as a function of temperature with the static magnetic field (H) parallel to the crystallographic directions [100] and [001], referred to the orthorhombic (Pbnm) axes. Detailed angular variations at constant temperature for H rotating in the (001), (010) and (-110) planes were also performed. The temperature dependence of the linewidth for fixed directions clearly reflects the changes in the symmetry of the crystal. At the Jahn Teller (JT) transition, a very noticeable decrease of the linewidth is observed. The angular dependence of the linewidth reveals an anisotropy that is temperature dependent. The amplitud of the variations decreases with T and almost dissapears above TJT ~ 280 K. However, there is a small remnant anisotropy present up to 400 K. We have carefully investigated the temperature dependence of this anisotropy across the Jahn-Teller transition. We showed that it is related with the evolution of the crystal field interactions. The symmetry of the ESR spectra is explained in terms of a spin Hamiltonian that includes isotropic Heisenberg superexchange, single ion-crystal field, and Dzyaloshinsky-Moriya antisymmetic exchange. The experimental angular variation of the ESR linewidth and its temperature dependence were fitted using the calculated expression for the linewidth as obtained from a perturbative Kubo-Tomita approach. From the ESR data we have obtained a ratio E/D = -0.4 for the spin Hamiltonian coefficients associated with the orthorhombic (E) and tetragonal (D) Jahn-Teller distortions of the MnO6 octahedra. The temperature dependence of the parameters of the model are presented, and the correlation with the dilatometry results are discussed. “High Temperature Susceptibility in Electron Doped Ca 1-x Y x Mn O 3 : Double Exchange versus Superexchange” H. Aliaga, M. T. Causa, M. Tovar, A. Butera, B. Alascio, D. Vega, A.G. Leyva, G. Polla, P. Konig J. Phys.: Condens. Matter 15 (2003) 249-258 We present a study of the magnetic properties of the electron doped manganites Ca 1-x Y x MnO 3 (for 0 ≤ x ≥ 0.25) in the paramagnetic regime. For the less doped samples (x ≤ 0.1) the magnetic susceptibility, X (T), follows a Curie- Weiss (CW) law only for T > 450 K and, below this temperature, X -1 (T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn 3+ and Mn 4+ ions, competes with classical superexchange. For higher levels of doping (x ≤ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (T mo ) and a better description of X (T) was obtained by assuming full delocalization of the e g electrons. In order to explore the degree of delocalization as a function of T and x, we analysed the problem through Monte Carlo